In response to a disruption or disturbance in an HVDC system, it may be necessary to isolate some equipment, such as cables, overhead lines and/or converter stations. The resulting outage(s) may or may not impact the operation of the HVDC system. If the operation is impacted, the HVDC system needs to be moved from an unstable, uneconomical and/or emergency operating point to a stable, economical and non-emergency operating point. The converter station local controls may attempt to restore the power balance in the direct current (DC) grid by changing the DC voltages to achieve a power balance in the lines terminating in the converter station. However, these uncoordinated actions, which are based on local measurements at the converter stations, might not drive system voltages to nominal levels. Rather, the combined individual actions of the converter station local controls might cause the HVDC system to experience prolonged operation at voltages above or below nominal, which may be unstable, uneconomical and/or detrimental to the security of the DC grid.
Examples of HVDC systems, and methods and systems for controlling HVDC systems, are disclosed in U.S. Pat. Nos. 4,419,591; 6,400,585 and 7,729,142, and in U.S. Patent Application Pub. Nos. 2006/0282239 and 2009/0279328. Examples of multi-terminal HVDC systems are disclosed in U.S. Pat. Nos. 4,419,591 and 7,729,142. The disclosures of these and all other publications referenced herein are incorporated by reference in their entirety for all purposes.